The mystery of tick-borne relapsing fever

As part of my ongoing efforts to ensure that discussions about tick-borne diseases in Canada don’t begin and end with Lyme disease, today I’m going to talk about tick-borne relapsing fever (TBRF), a relatively rare illness in Canada that’s also known as relapsing tick fever.

TBRF is one of the oldest known tick-borne illnesses in this country, with documented cases in the BC Interior stretching as far back as the early 1930s. Those cases were unlikely to have been the first ones to manifest in the province, but they were the first that caught the attention of doctors who realized they were seeing something unique.

TBRF is something of an enigma. Despite being one of the earliest recognized of the tick-borne illnesses in Canada, it often goes unrecognized and is tragically under-researched so there remains a staggering amount we still don’t know about it. But there’s one thing that we do know: TBRF is caused by infection with a bacterium (Borrelia hermsii) closely related to the species that cause Lyme disease (Borrelia burgdorferi sensu lato). However, unlike Lyme bacteria, which is transmitted in BC primarily by Western blacklegged ticks (Ixodes pacificus) and to a lesser extent by Ixodes angustus ticks (no common name), TBRF is thought to be transmitted by night-feeding, soft-bodied Ornithodorus hermsi ticks.

I say "thought to be" because although TBRF has long been associated with Ornithodorus hermsi ticks in the US, no tick in BC of any species has ever been found to be infected with Borrelia hermsii despite an almost 100 year history of humans acquiring the bacterium within the province and efforts by scientists to capture infected ticks on those rare occasions when outbreaks have occurred. That leaves a very big question as to whether Ornithodorus hermsi really is the primary agent of TBRF in BC. It's a question that currently does not have an answer.

Virtually all cases of TBRF documented in Canada were contracted in BC’s southern interior, most notably in the Thompson-Okanagan and Kootenay-Boundary regions. But public health officials don’t really know TBRF's full range partly due to the illness often going undiagnosed or being misdiagnosed and partly due to the failure to isolate Borrelia hermsii from any tick species in the province. Until someone can definitively say which ticks are infecting people with TBRF and doctors get better at recognizing it, public health officials can't definitively say where the bacteria is endemic. They can only say where cases have been identified.

A 2017 report put out by public health officials in BC noted there are a surprisingly high number of TBRF cases in the province when compared with other regions in the Pacific Northwest where TBRF is also endemic. There are also a disproportionate number of people in BC simultaneously testing positive for both TBRF and Lyme disease. It’s possible that cross-reactivity between the two tests is playing a role. Cross-reactivity can happen when indirect antibody tests are used to test for the presence of organisms that produce similar antibodies — such as two different species of borrelia — causing the tests to come up positive for the real pathogen and the similar one, making it appear as though someone has both diseases when really they have one infection that has reacted on both tests.

That may be all that’s happening in BC. But enough researchers have noted the curious double positives that it has raised an important question: Are British Columbians contracting TBRF and Lyme disease from a single tick bite?

That shouldn’t be possible. If Lyme disease and TBRF truly are carried by two very different types of ticks as is the current belief then for someone to contract both infections simultaneously would mean they would've had to have been bitten by a single tick harbouring two infections, both of which are considered rare in BC. That would also mean Borrelia burgdorferi and Borrelia hermsii would share a common host that has yet to be identified.

There are several suspects.

When those initial cases of TBRF were reported in the 1930s, doctors at the time felt the bacterium was being transmitted by Rocky Mountain wood ticks (Dermacentor andersoni), which were -- and still are -- ubiquitous in the province. A noted tick expert cast doubt on that assertion soon afterwards, but the possibility remains that those original doctors may have been right and Rocky Mountain wood ticks could be at least partially responsible for TBRF in BC. Other suspects include Western blacklegged ticks, Ixodes angustus, and several native tick species.

The sad fact is there hasn’t been enough Canadian research on TBRF for anyone to make a definitive statement on the subject. Instead we have to rely on studies that have been done in the US and insight gained from a century of tick research in BC that points to hazy possibilities. It should be noted that American research into TBRF is also not as robust as one would hope although there is certainly more of it there than in Canada.

Still, if there is one thing we have learned in the past decade of tick-borne illness research in Canada, it’s that the situation in this country doesn’t always mirror the situation in the US. So the tick responsible for TBRF in Canada may not be the same one responsible for US cases. Time will tell.

Originally published in The Lyme Report, Issue 28.

Photo Credit: National Institute of Allergy and Infectious Diseases (NIAID)